Currently known secondary batteries using an aqueous alkali solution as an electrolyte solution include nickel-metal hydride batteries, nickel-cadmium batteries, nickel-iron batteries, and nickel-zinc batteries. Nickel hydroxide is used for a positive electrode of such an alkaline secondary battery. In the case of a nickel-cadmium battery, a mixture of metal cadmium and cadmium hydroxide is used for a negative electrode. In the case of a nickel-iron battery, a mixture of metal iron and iron hydroxide is used for a negative electrode. In the case of a nickel-zinc battery, a mixture of metal zinc and zinc hydroxide is used for a negative electrode. In the case of a nickel-metal hydride battery, a hydrogen storage alloy is used for a negative electrode. In the case of an alkaline secondary battery, an aqueous alkali solution, such as a caustic potash solution or a caustic soda solution in which lithium hydroxide is dissolved, is usually used as an electrolyte solution.
Among these secondary batteries, in the case of a nickel-cadmium battery, a nickel-iron battery, and a nickel-zinc battery, dissolution and redeposition reactions occur at the negative electrode when charging and discharging are performed. Therefore, these batteries are inferior in terms of power capability. In the case of a nickel-zinc battery, a negative electrode active material is deposited in a dendritic form at the time of redeposition. Accordingly, nickel-zinc batteries are short-lived and have a risk of short-circuiting. Although nickel-cadmium batteries were wide spread, their capacity per volume is approximately half of that of nickel-metal hydride batteries. Moreover, effects of cadmium on the human body have been an issue of concern. Furthermore, the discharge voltage of nickel-cadmium batteries is the same as that of nickel-metal hydride batteries. For these reasons, nowadays nickel-cadmium batteries have been almost entirely replaced by nickel-metal hydride batteries.
Charging and discharging reactions in an alkaline electrolyte solution of a nickel-metal hydride battery can be represented by formulas shown below. In the formulas, M represents a metal element (hydrogen storage alloy).Positive Electrode: Ni(OH)2+OH−NiOOH+H2O+e−  [Formula 1]Negative Electrode: M+H2O+e−MH+OH−  [Formula 2]All reactions: Ni(OH)2+MNiOOH+MH  [Formula 3]
During charging, at the positive electrode, nickel hydroxide desorbs hydrogen, and oxy nickel hydroxide is formed. At this time, the metal (hydrogen storage alloy) of the negative electrode absorbs hydrogen that is produced through electrolysis of water, and becomes a hydride. On the other hand, during discharging, hydrogen is desorbed from the metal of the negative electrode, and electricity is generated together with water.
Nickel-metal hydride batteries have high-power capability and realize stable charging and discharging. Therefore, nickel-metal hydride batteries are widely used in household electrical appliances, mobile devices such as mobile phones and laptop PCs, and rechargeable power tools. Nickel-metal hydride batteries are expected to be utilized as an emergency power supply for facilities such as factories or hospitals where reliability is considered as the most important feature of the emergency power supply. The primary object of using an emergency power supply is to prevent devices from stopping when power failure has occurred, by discharging previously charged electric power. Therefore, such an emergency power supply needs to be always fully charged and ready for use.
Accordingly, a secondary battery to be used as such an emergency power supply is not a high-rate charging type battery which is fully charged within a short period of time and thereafter the charging is stopped, but is a battery capable of maintaining its capacity at a particular level or higher after being charged. Such a battery employs, for example, a charging method in which after the battery is fully charged, charging is continued with a weak electric current to compensate for the battery's self-discharge (i.e., trickle charging), or a charging method in which when the battery is fully charged, a current flows through a bypass circuit within a battery charger so that a load on the battery is reduced to zero (i.e., float charging).
When the battery is overcharged, oxygen gas is evolved from the positive electrode through a reaction shown below (represented by Formula 4). A large part of the evolved oxygen reacts with hydrogen at the surface of the negative electrode, and thereby water is produced as shown in Formula 5. M represents a metal element (hydrogen storage alloy). Accordingly, nickel-metal hydride batteries are formed such that the discharge capacity of the negative electrode is equivalent to or higher than the discharge capacity of the positive electrode, i.e., the battery capacity is limited by the positive electrode.Oxygen Evolution (Positive Electrode): OH−½H2O+¼O2+e−  [Formula 4]Oxygen Absorption (Negative Electrode): MH+¼O2M+½H2O  [Formula 5]All Reactions: M+H2O+e−MH+OH−  [Formula 6]
However, a part of the evolved oxygen oxidizes the hydrogen storage alloy, causing degradation of the negative electrode, resulting in a decrease in the hydrogen absorption/desorption rate and the chargeable capacity of the negative electrode. In particular, if a secondary battery is charged under a high-temperature atmosphere, then charging efficiency is reduced, resulting in lower battery capacity than in a case where the secondary battery is charged under an ordinary temperature. The reason for this is that under a high-temperature condition, an oxygen evolution potential is lowered and the oxygen evolution reaction shown in [Formula 4] occurs prior to the charging reaction shown in [Formula 1]. An increase in battery voltage, an increase in battery temperature, or differential values of those increases with reference to time, is usually used to detect the end of charging of a battery. However, there is a drawback that such a method does not always work accurately depending on the usage environment of the battery.
Emergency power supplies are assumed to be used under a wide range of temperatures. Therefore, it is necessary to improve the aforementioned charging efficiency under high temperatures. Also, if an overcharged state is kept for a long period of time as in the case of float charging, then the amount of oxygen evolution from the positive electrode increases, which increases a risk of oxidation degradation of the surface of the negative electrode. If such oxidation degradation of the surface of the negative electrode occurs, the hydrogen absorption/desorption capability and charging capacity of the negative electrode are reduced. There is disclosed an effective method of suppressing the oxygen evolution under a high temperature (Non Patent Literature 1), in which method the surface of a nickel positive electrode is coated with a hydroxide containing yttrium, calcium, or cobalt. However, the potential of these compounds is lower than that of nickel hydroxide, and the coating of such a compound tends to cause a decrease in battery potential. For this reason, a negative electrode material that is not easily oxidized is sought after.
In a case where a secondary battery is installed in a vehicle or a factory facility, it is assumed that the secondary battery is used under a wider range of temperatures than in a case where the secondary battery is applied in a household electrical appliance. Moreover, in such a case, the battery to be installed is large-sized, and accordingly, it is considered that the cost of replacing the battery is high. Therefore, the battery is required to be durable enough to withstand a long-term use (longer than 10 to 20 years) under a high-temperature environment. Also in this respect, a negative electrode material that is not easily oxidized is sought after.
LaNi5-based alloys and La—Mg—Ni based superlattice alloys that are widely used in nickel-metal hydride batteries contain expensive elements such as rare earth elements. The use of such alloys is a contributing factor to the high cost of fabricating a negative electrode and to the high cost of fabricating the entire battery. Since these elements are unevenly distributed resources, it is necessary to use a universal resource as an electrode material from the standpoint of stable resource supply.
Alkaline secondary batteries in which not an easily oxidized hydrogen storage alloy but an inoxidizable oxide is used for an electrode are currently under consideration. For example, Non Patent Literature 2 discloses a rechargeable aqueous lithium ion battery in which MnO2 and carbon are used for a positive electrode and a negative electrode, respectively, and an electrolyte solution contains a lithium compound. However, the aqueous lithium ion battery disclosed in Non Patent Literature 2 exhibits a rapid capacity decrease within 10 cycles of charging and discharging. Thus, it is considered that the aqueous lithium ion battery disclosed in Non Patent Literature 2 has no practical use.
Patent Literature 1 discloses a rechargeable aqueous lithium ion secondary battery in which a lithium manganese oxide or a lithium vanadium oxide is used for a positive electrode and a negative electrode, respectively, and an electrolyte solution in which a lithium salt is dissolved is used. However, a current used for charging and discharging is 1 mA/g, which is significantly small, and the battery degrades after 20 to 30 cycles of charging and discharging. Thus, it is considered that the aqueous lithium ion battery disclosed in Patent Literature 1 has no practical use.
Patent Literature 2 discloses an aqueous lithium ion secondary battery in which: a combination of two kinds of lithium intercalation compounds is used, the lithium intercalation compounds having different charging/discharging potentials of 3.4 V or higher (e.g., LiFePO4: 3.45 V) and 2.2 V or lower (e.g., Li4Ti5O12), respectively; and an aqueous solution in which a lithium salt is dissolved and of which the pH is 14 or higher is used as an electrolyte solution. Patent Literature 3 discloses an aqueous secondary battery in which NiO2, CoO2, Mn3O4, MnO2, VO2, V2O5, MoO2, or WO3 is used as an active material.
However, the secondary batteries disclosed in Patent Literature 2 and Patent Literature 3 require lithium ion intercalation/deintercalation reactions to occur. Accordingly, the high-rate discharge capability and cycle-life performance of these batteries are poor. The reason for this is as follows: since a lithium ion is larger in size than a hydrogen ion, the diffusion rate of lithium ions is slow and changes in electrode volume caused by lithium ion insertion and extraction are great.
Patent Literature 4 discloses a secondary battery including a fiber electrode that is obtained by forming a thin active material layer around each of very thin fibrous current collectors. The fiber electrode of the secondary battery has a completely different structure from that of electrodes of conventional secondary batteries. Such a structure makes it possible to greatly improve the high-power capability of the battery. The fiber electrode has a significantly large surface area, which allows charging and discharging to be performed with a high current density. The charging/discharging speed can be greatly improved by forming both positive and negative electrodes into such fiber electrodes. In order to fabricate such a fiber electrode, it is necessary to use a negative electrode material that allows an active material layer to be readily formed on a very thin fibrous current collector.